Multiple-input multiple-output (MIMO) employs multiple antennas at the transmitter and/or at the receiver to significantly improve the data throughput and transmission reliability. The data throughput can be increased at the link level and/or at the system level. Both spatial multiplexing and beamforming can be used to improve the spectral efficiency and the data throughput. Spatial multiplexing is very effective in boosting the link level throughput and the peak rate since it allows multiple data streams to be transmitted simultaneously to the same user through parallel channels. Spatial multiplexing is suitable when spatial correlation between antennas (both at the transmitter and the receiver) is low. Beamforming or precoding is able to boost the signal to interference plus noise ratio (SINR) of the channel, and thus the channel rate. Essentially, precoding involves applying proper weights over multiple transmit antennas and the weight calculation can be based on either channel reciprocity or via feedback.
MIMO has been extended to multi-cell operations, the so-called coordinated multi-point processing (CoMP), a key study item in LTE-Advanced as described in 3GPP TR 36.814, v2.0.0, “Further Advancements for E-UTRA, Physical Layer Aspects,” March 2010. The concept is to take advantage of increased spatial dimensions by performing either coordinated beamforming/scheduling or joint transmission over multiple transmit antennas from neighboring cells/nodes. Coordinated beamforming/scheduling does not need to share the transmitted data among neighboring cells/nodes, thus its requirement for the backhaul is much lower. Joint transmission has more potential for the capacity improvement albeit with more burden on the backhaul.
Joint processing and coordinated scheduling/beamforming can be illustrated in a generic model as seen in FIG. 1 where the cooperating set consists of two neighboring base stations, namely, (evolved node B) eNB1 and eNB2. Two user equipment (UE) locations are denoted by UE1 and UE2, and are in the CoMP serving area. For simplicity (without losing the generality), each UE shown in FIG. 1 has only one receive antenna and each base station has two transmit antennas. The convention “hi,j;u,” is used to denote the complex coefficient of the channel connecting j-th antenna of i-th eNB and u-th UE, assuming single-path fast fading. Out of all combinations, there are a total of eight channel coefficients in this two-site CoMP.
In the case of joint transmission, data is available at both eNB1 and eNB2 to achieve the inter-site precoding gain. The transmit weight applied at j-th antenna of i-th eNB for u-th UE is denoted as “wi,j;u”. The optimum weight vector is chosen from the total candidate set W4 (the subscript 4 means that each vector has four elements). One criterion can be maximizing the inner product of the weight vector and the channel vector for the same UE, or mathematically:
                              w                      u            ,            JT                          =                                            [                                                                                          w                                              1                        ,                                                  1                          ;                          u                                                                                                                                                                                w                                              1                        ,                                                  2                          ;                          u                                                                                                                                                                                w                                              2                        ,                                                  1                          ;                          u                                                                                                                                                                                w                                              2                        ,                                                  2                          ;                          u                                                                                                                                ]                        JT                    =                                                    arg                ⁢                                                                  ⁢                min                                                              w                                      u                    ,                    JT                                                  ∈                                  W                  4                                                      ⁢                                          ∑                                  i                  ,                                      j                    =                    1                                                  2                            ⁢                                                w                                      i                    ,                                          j                      ;                      u                                                                      ⁢                                  h                                      i                    ,                                          j                      ;                      u                                                                                                                              (        1        )            
For multi-user joint transmission, UEs should be paired such that the co-channel interference can be minimized. That is as follows:
                                          {                                          u                ⁢                                                                  ⁢                1                            ,                              u                ⁢                                                                  ⁢                2                                      }                                pair            ,            JT                          =                                            arg              ⁢                                                          ⁢              min                                                      u                ⁢                                                                  ⁢                1                            ,                                                u                  ⁢                                                                          ⁢                  2                                ∈                U                                              ⁢                                    ∑                              i                ,                                  j                  =                  1                                            2                        ⁢                                          w                                  i                  ,                                      j                    ;                                          u                      ⁢                                                                                          ⁢                      1                                                                                  ⁢                              w                                  i                  ,                                      j                    ;                                          u                      ⁢                                                                                          ⁢                      2                                                                      *                                                                        (        2        )            
In the case of coordinated scheduling/beamforming, user data is available only at the serving cell and there is no inter-site precoding gain. Transmit weight is chosen from the total candidate vector set W2 (the subscript 2 means that each vector has two elements). One simple way would be (without jointly optimizing weight vectors between sites/nodes):
                              w                      u            ,                          CS              /              BF                                      =                                            [                                                                                          w                                              u                        ,                        1                                                                                                                                                        w                                              u                        ,                        2                                                                                                        ]                                      CS              /              BF                                =                                                    arg                ⁢                                                                  ⁢                max                                                              w                                      u                    ,                                          CS                      /                      BF                                                                      ∈                                  W                  2                                                      ⁢                                          ∑                                  j                  =                  1                                2                            ⁢                                                w                                      u                    ,                    j                                                  ⁢                                  h                                      u                    ,                                          j                      ;                      u                                                                                                                              (        3        )            
Similar to multi-user joint transmission, UEs in CoMP area sharing the same resource need to be carefully paired in order to reduce the other cell interference. That is as follows:
                                          {                                          u                ⁢                                                                  ⁢                1                            ,                              u                ⁢                                                                  ⁢                2                                      }                                pair            ,                          CS              /              BF                                      =                                            arg              ⁢                                                          ⁢              min                                                      u                ⁢                                                                  ⁢                1                            ,                                                u                  ⁢                                                                          ⁢                  2                                ∈                U                                              ⁢                                    ∑                              j                =                1                            2                        ⁢                                          w                                                      u                    ⁢                                                                                  ⁢                    1                                    ,                  j                                            ⁢                              w                                                      u                    ⁢                                                                                  ⁢                    2                                    ,                  j                                *                                                                        (        4        )            
The difference between coordinated scheduling and coordinated beamforming lies in the spatial correlation assumption between the transmit antennas on each eNB. Coordinated scheduling assumes widely spaced vertical or cross-polarized antennas, whereas coordinated beamforming implies highly correlated antenna array to actually form the physical beams in each cell.
The study item of CoMP will continue in Release 11. An important aspect of CoMP is the spatial channel state information (CSI) feedback. Given the different geometries experienced by each UE, especially at cell edges where CoMP is mostly targeted for, the number measurement sets for different UEs can have different number of cells. These and other circumstances present problems and obstacles that are overcome by the method and system of the present invention as hereinafter described.